When I design I usually start super simple: What do I want to make. By this time, I basically know my aesthetic so I know how I want whatever I'm making will look but the various elements can be pretty complicated.
If I wanted to make a bag, I want to know what kind of bag it'll be, what kinds of enclosures I want, how the bag will be held or carried and pockets. Don't forget the pockets. I'll usually have an idea about the dimensions and from there I break down each sewable element such as which step needs to follow the previous step, measuring panels with seam allowances, etc. Often I'll look online for how construction works, the materials needed, and over the course of a few hours to a few weeks, I will spend designing how I want this bag to look and how I'd assemble it.
I have a fairly tame aesthetic and tend to want as simple a design as possible, since I don't work with a lot of exotic materials/tools. Also I find that busy designs are needlessly complicated and there are often too many ways things could go to hell. Remember KISS: Keep It Simple Stupid.